1. Field of the Invention
The present invention relates to a sheet feeder provided to an image forming apparatus, and more particularly, relates to a sheet feeder that feeds a sheet from a feed tray and an image forming apparatus provided with the sheet feeder.
2. Description of the Related Art
In a sheet feeder provided on an image forming apparatus and the like, it is necessary to send out a plurality of sheets such as recording sheets stacked in a feed tray one by one accurately. This requires a separation mechanism to separate the stacked sheets one by one. For the separation mechanism, a frictional separation method is widely used in which a sheet sent from a feed tray by a pickup roller is separated and fed by frictional force. Examples of the frictional separation type separation mechanism include a combination of a separation roller and a friction pad, a combination of a separation roller and a reverse roller, and the like.
As a sheet feeder provided with a separation mechanism different from the friction separation type, Japanese Patent Application Laid-open No. 2007-45630 discloses a conventional sheet feeder that attracts a sheet on a conveying member by a negative pressure to separate and feed the sheet.
In the conventional sheet feeder, a sheet suction unit is located inside a sheet conveyor belt as an endless belt. The sheet conveyor belt is disposed at a position facing the topmost sheet on top of a plurality of stacked sheets in a feed tray. The conventional sheet feeder is further provided with a blower device that blows air toward the leading edge of the sheets in the feed tray.
In such a sheet feeder, when a feed command is received from an image forming apparatus, operations of a suction device that generates a negative pressure in the sheet suction unit and the blower device are started while the sheet conveyor belt is being stopped. The blower device in operation blows air toward the leading edge of the sheets in the feed tray, which separates and lifts the topmost sheet from the other sheets below. A countless number of suction holes are formed over the entire sheet conveyor belt. Operating the suction device generates a negative pressure in the sheet suction unit, and the negative pressure acts below the sheet conveyor belt through the suction holes, whereby the topmost sheet lifted is attracted on the sheet conveyor belt.
After an elapse of a given period of time from the start of the operations of the suction device and the blower device, an endless movement of the sheet conveyor belt is started while the suction device and the blower device are in operation. Consequently, the topmost sheet attracted on the sheet conveyor belt is conveyed toward a subsequent step, and then an image is formed.
After the first topmost sheet is conveyed, the second topmost sheet subsequently positioned at the top is attracted on the sheet conveyor belt when the trailing edge of the first topmost sheet starts passing through the suction area of the sheet suction unit. Consequently, immediately after the second topmost sheet is attracted on the sheet conveyor belt, the leading edge of the second topmost sheet and the trailing edge of the first topmost sheet overlap each other. Therefore, if the endless movement of the sheet conveyor belt is successively continued, the second topmost sheet may be conveyed together with the first topmost sheet, resulting in so-called double feed. Accordingly, when it is detected that the topmost sheet reaches the position where a conveyance force is applied by a downstream conveying member located on the downstream of the sheet conveyor belt in the sheet conveying direction, the endless movement of the sheet conveyor belt is stopped. In this case, the downstream conveying member is driven while the sheet conveyor belt is being stopped. As a consequence, while the first topmost sheet is being conveyed continuously, the conveyance of the second topmost sheet that is attracted on the sheet conveyor belt when the trailing edge of the first topmost sheet starts passing through the suction area is stopped, whereby double feed is prevented.
Thereafter, according to a predetermined feeding interval, the endless movement of the sheet conveyor belt is resumed after an elapse of a predetermined time from the time the sheet conveyor belt is stopped. Accordingly, the second topmost sheet attracted on the sheet conveyor belt is conveyed toward the subsequent step similarly to the first topmost sheet.
While the blower device, the suction device, and the downstream conveying member are in operation, the endless movement of the sheet conveyor belt is controlled ON and OFF, whereby the sheets in the feed tray are sequentially fed one by one toward an image forming unit.
When the feeding of a specified number of sheets is completed, the operations of the blower device, the suction device, the sheet conveyor belt, and the downstream conveying member are stopped to end the feeding operation performed by the sheet feeder.
In the sheet feeder that attracts the sheet on the conveying member by a negative pressure, stopping the operations of the blower device, the suction device, the sheet conveyor belt, and the downstream conveying member stops the feeding of sheets to the subsequent step. However, the negative pressure in the sheet suction unit is not released immediately. After the suction device is stopped, the negative pressure in the sheet suction unit gradually approaches the atmospheric pressure. Therefore, at the time the feeding operation ends, the suction power for an end-of-feeding topmost sheet, i.e., a sheet positioned on the top in the feed tray when the feeding ends, remains to be held.
When the negative pressure in the sheet suction unit gradually approaches the atmospheric pressure and the self weight of the sheet exceeds the suction power for the end-of-feeding topmost sheet, the end-of-feeding topmost sheet falls onto a bundle of sheets in the feed tray by its own weight. When the end-of-feeding topmost sheet is of a lightweight such as a small thin sheet, it may take several tens of seconds for the sheet to fall onto the bundle of sheets in the feed tray after the feeding operation ends.
The sheet feeder is designed to allow the feed tray to be drawn out from the body to replenish the feed tray that has lost sheets by feeding or to replace sheets. When drawing out the feed tray from the body, if the end-of-feeding topmost sheet is held attracted on the sheet conveyor belt, the sheet may be damaged or the sheet may fall into an area inside the device where a user cannot reach.
The user is aware that the sheet is held attracted on the belt when the feed tray is drawn out immediately after the feeding ends based on experience or from a notice given in a user's manual. Consequently, the user draws out the feed tray after waiting for some time since the feeding ends.
Therefore, when drawing out the feed tray, the user waits for the end-of-feeding topmost sheet that is previously loaded to fall onto the bundle of sheets in the feed tray and then draws out the feed tray. This may make the user wait for several tens of seconds after the feeding operation ends, thereby causing a large waste of time each time sheets are replenished or replaced.